1. Field of the Invention
The present invention relates to a method used in a communication system, and more particularly, to a method of uplink control information (UCI) transmission in a communication system.
2. Description of the Prior Art
The 3rd Generation Partnership Project (3GPP) has initiated the Long Term Evolution (LTE) program to bring novel network architecture and configuration, novel technology, and novel applications and services to provide reduced latency, improved spectral efficiency and faster user experiences with less cost. An LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of a base station, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint (CoMP) transmission/reception, uplink (UL) multiple-input multiple-output (MIMO), etc.
CA is a mechanism to increase bandwidth of a wireless communication system. This feature allows scalable expansion of effective bandwidth delivered to a user terminal through concurrent utilization of radio resources across multiple serving cells or component carriers (CCs). A communication device or user equipment (UE) with reception and/or transmission capabilities for CA may simultaneously receive and/or transmit on multiple CCs corresponding to multiple serving cells. To support the future growth of mobile traffic data, 3GPP has approved to extend CA up to 32 CCs in Rel-13 standard. Nevertheless, the extension of the CA framework beyond 5 CCs would bring serious challenges as all the CA aspects do not scale as the number of aggregated CCs increases.
For example, extending downlink CA for up to 32 DL CCs increases the amount of uplink control information (UCI) transmitted on a physical uplink control channel (PUCCH) from a UE to a base station significantly. The UCI primarily includes channel status information (CSI) and hybrid automatic repeat request acknowledgment (HARQ-ACK). Besides, there is only one single PUCCH on an uplink CC (i.e., a primary CC). The UCI corresponding to all downlink CCs can only be transmitted on the uplink CC irrespective of the number of downlink CCs according to current specifications. In legacy technology, only one CSI report can be transmitted over the PUCCH. If more than one CSI reports are scheduled to be transmitted via the same uplink CC, a considerably large payload would need to be support. The payload size of the PUCCH is however fixed and limited—even the latest PUCCH format 3 can merely have a 22-bit payload size to the maximum.
In addition, if the UE is scheduled to transmit at least one CSI report over a PUCCH on an uplink CC (e.g., the primary CC) and at least one CSI report over another PUCCH on another uplink CC (e.g., a secondary CC), there is a trade-off between uplink coverage and CSI transmission because the uplink transmission power of a communication device is also limited. Basically, physical random access channel (PRACH) power is first prioritized, and remaining power may be used by the PUCCHs. When the UE does not have enough power, power is then scaled down on the PUCCHs, which could lead to dramatic drop in signal-to-noise ratio (SNR).
Therefore, an improved method of reporting UCI over a pre-configured uplink resource (e.g., PUCCH) is required particularly when a UE is configured with a large number of serving cells.